1. Field of the Invention
The present invention relates generally to pressurized cylinder-and-piston rod devices, such as gas springs, and in particular to improved structure for use with such devices to retain the piston rod in a terminal position within the cylinder.
2. The Prior Art
A prior art cylinder-and-piston rod device of the general type referred to is disclosed in U.S. Pat. No. 4,045,008, issued Aug. 30, 1977, and in the corresponding German Auslegeschrift No. 25 16 478, published Apr. 20, 1978.
In the known device, the retaining structure includes a retaining piston mounted on the piston rod assembly as an axial extension thereof and a retaining cylinder provided at the closed end of the cylinder in a fixed position relative to the cylinder. The retaining piston and the retaining cylinder together define a retaining chamber when the retaining piston has entered the retaining cylinder, which retaining chamber is connected to the larger cylinder cavity by a passage that is provided with a check valve for controlling fluid flow between the retaining chamber and the cylinder cavity. When the retaining piston has entered the retaining cylinder, a retaining action is exerted on the piston rod, via the retaining piston, which acts in a direction opposite to the outward biasing force of the pressurized fluid on the piston rod. The piston rod is thereby held against returning to the outer terminal position as long as no external force is applied to the piston rod tending to withdraw the piston rod assembly towards its outer terminal position. In this device, the retaining action on the piston rod assembly occurs only when, after having been introduced into the retaining cylinder, the piston rod has moved outward thereof for a short distance so as to reduce the pressure within the retaining chamber. It is not possible in this device to modify the retaining action exerted by the retaining structure in such a way as to prevent such outward movement of the piston rod assembly once the retaining piston has entered the retaining cylinder, i.e., once the retaining structure has been engaged.